MB-11461 Use a single upr message for gathering vbucket-seqno stats

The view engine has to get vbucket sequence numbers for all partitions
whenever a query is processed. The time taken for requesting the stats
and collecting partition sequence numbers affect the query latency
during stale=update_after and stale=false queries. Currently we are
requesting one message per partition to the EP-Engine. That means, for
1024 partitions, we will need to process 1024 upr messages.

Added support for sending single upr vbucket-seqno stats message and
parse interested partition seqnos. This should reduce the network
noise caused by stats requests and improve query latency.

Change-Id: I602e11923fd26980e1f08dab836b4dfa19a02eb3
Reviewed-on: http://review.couchbase.org/38691
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: buildbot <build@couchbase.com>